The amplifier of the invention has the following features: (1) Uniform gain from direct-current to frequencies greater than 100 MHz; (2) High power and low distortion in a direct-current coupled amplifier; (3) Input and output impedances which do not vary as a function of frequency to any material extent from a desired impedance (to be referred to herein as "low VSWR"); (4) A direct-current coupled amplifier having gain with a single pole roll-off and without any tendencey to peak so that the amplifier may be used for amplifying pulses; (5) Stable direct-current and alternating-current performance in such an amplifier over a wide temperature range; (6) A direct-current coupled amplifier whose input and output are centered around zero volts D.C.; and (7) A low noise figure.
Prior to the present invention, the only means for achieving all the features listed above was to place an alternating-current coupled high frequency amplifier in parallel with a direct-current coupled low frequency amplifier. Then, the frequency response of each of the two amplifiers was carefully adjusted in an attempt to provide uniform response over a wide frequency range. A low VSWR was then achieved by properly padding the input and the output of each of the two amplifiers and by then summing the two inputs and the two outputs.
The major disadvantages encountered in the prior art arrangement described in the preceding paragraph are the difficulty in obtaining overall uniform response over the frequency range of the two amplifiers, the fact that padding and summing the outputs of the two amplifiers to produce low VSWR attenuates the overall output materially thereby reducing the gain and efficiency of the system, the phase non-linearity of the output of the two amplifiers with respect to the input, the deviations of the performance of most commonly used AC coupled amplifiers as a function of temperature, and the increased noise figure resulting from padding the input.
A significant advantage of the direct-current coupled amplifier of the present invention over the prior art amplifiers described above is that the amplifier of the present invention is a single amplifier which exhibits good direct-current characteristics as well as a good alternating-current response, so that the achievement of improved response and low VSWR, as well as high gain and high efficiency becomes feasible. In addition, the phase relationship of the output of the amplifier of the invention with respect to the input is extremely linear over the major portion of the bandwidth of the amplifier.